Preparation of wide gap Cu(In,Ga)S2 films on ZnO coated substrates

We have prepared Cu(In,Ga)S₂ films at growth temperatures from 300 °C to 580 °C with a homogeneous gallium depth distribution (estimated band gap 1.67 eV) onto soda lime glass (SLG) substrates with one of three different kinds of back contact: Mo(1000 nm), ZnO(500 nm), and Mo(30 nm)/ZnO(500 nm), respectively. We have also investigated the depth profiles of Zn and Na (diffused from SLG) in Cu(In,Ga)S₂ films by secondary ion mass spectroscopy (SIMS). The efficiency of solar cells on Mo increases with increasing growth temperature. It is higher on Mo/ZnO than on ZnO, and increases from 350 °C to 450 °C, then decreases above 450 °C. It was observed by SIMS that the amount of Zn in Cu(In,Ga)S₂ on Mo/ZnO is lower than it is on ZnO up to 450 °C, and a large amount of Zn diffuses into absorbers over 450 °C, which contributes to decreasing efficiency. The amount of Na in the back contact increases with growth temperature. The depth distribution of Na in Cu(In,Ga)S₂ films on Mo is almost constant in the order of 10¹⁷-10¹⁸ cm^− 3, on ZnO and Mo/ZnO the Na concentration increases towards the surface and is in the range of 10¹⁵-10¹⁷ cm^− 3.     

Electron transport in InGaO3(ZnO)m (m=integer) studied using single-crystalline thin films and transparent MISFETs

We have investigated the characteristics of transparent metal-insulator-semiconductor field-effect transistors (MISFETs) fabricated using InGaO₃(ZnO)_m (m=integer) single-crystalline thin films as n-channel layers and amorphous alumina as gate insulator films. The MISFETs exhibit good characteristics such as insensitivity to visible light illumination, off-current as low as ∼1 nA with a positive threshold voltage of ∼3 V and on/off current ratio of 10⁵. The field-effect mobility increased from ∼1 to ∼10 cm² (V s)⁻¹ as the m-value increased. Room temperature Hall mobility also increased. However, unexpectedly these values were lower than the field-effect mobility. It is explained by existence of shallow localized state in the homologous compounds.     

Thermally stable, highly conductive, and transparent Ga-doped ZnO thin films

Highly conductive and transparent films of Ga-doped ZnO (GZO) have been prepared by pulsed laser deposition using a ZnO target with Ga₂O₃ dopant of 3 wt.% in content added. Films with resistivity as low as 3.3 × 10^− 4 Ω cm and transmittance above 80% at the wavelength between 400 and 800 nm can be produced on glass substrate at room temperature. It is shown that a stable resistivity for use in oxidation ambient at high temperature can be attained for the films. The electrical and optical properties, as well as the thermal stability of resistivity, of GZO films were comparable to those of undoped ZnO films.     

Characterization of ZnO:Al/Au/ZnO:Al trilayers for high performance transparent conducting electrodes

Symmetric ZnO:Al/Au/ZnO:Al trilayers were sputter-deposited and characterized for transparent conducting electrodes, varying the thickness of the ZnO:Al (AZO) and Au layers. The optical transmission for normal light incidence is optimum for an AZO thickness of 50 nm, due to the suppression of reflection. In this case, the transmittance is more than 0.7 for wavelengths above 400 nm and for a Au thickness of 5 nm. At the same time, the sheet resistance is approx. 30 Ω, which can be decreased to 12 Ω with the increase of the Au thickness to 9 nm. This is achieved with a moderate loss in the optical transmission. The figure of merit for transparent conducting electrodes, as introduced by G. Haacke (J. Appl. Phys. 47 (1976) 4086) yields values from 29.4 × 10^− 3 to 6.9 × 10^− 3 Ω^− 1, depending on the Au thickness and the considered wavelength range.     

Synthesis of Tb3Al5O12 (TAG) transparent ceramics for potential magneto-optical applications

Tb₃Al₅O₁₂ transparent ceramics have been prepared by solid state reaction and vacuum sintering. The optical quality and the microstructure of the samples were investigated. The sample sintered at 1650 °C possessed relatively good optical transparency from 400 nm to 1600 nm. The Verdet constant measured at 632.8 nm of the quasi-pore-free Tb₃Al₅O₁₂ transparent ceramic was −172.72 rad T⁻¹ m⁻¹, which was close to the counterpart of Tb₃Al₅O₁₂ single crystal. The thermal conductivity of the sample was also measured. To the best of our knowledge, this is the first time that Tb₃Al₅O₁₂ transparent ceramic with relatively good optical quality and magneto-optical property has been reported.     

ZnO and Zn1−xCdxO nanocrystallites obtained by oxidation of precursor Langmuir-Blodgett multilayers

ZnO and Zn_1−xCd_xO nanocrystallites were prepared by oxidation of zinc arachidate-arachidic acid and zinc arachidate-cadmium arachidate-arachidic acid LB multilayers, respectively. The metal content of the multilayers was controlled by manipulation of subphase composition and pH. Precursor multilayers were oxidized in the temperature range of 400 °C-700 °C. The formation of ZnO and Zn_1−xCd_xO was confirmed by UV-Visible spectroscopy. Uniformly distributed, isolated and nearly mono-dispersed nanocrystallites of ZnO (11 ± 3 nm) and Zn_1−xCd_xO (18 ± 6 nm) were obtained.     

Transparent glass ceramic containing Er:CaF2 nano-crystals prepared by sol-gel method

Er³⁺ doped SiO₂-CaF₂ transparent glass ceramic was prepared by sol-gel method. The microstructural evolution of the samples was studied with X-ray diffraction (XRD), transmission electron microscopy (TEM), and infrared spectra (IR). After heat-treatment at 900 °C, the Si-OH bonds and other organic groups were basically eliminated. The CaF₂ crystallites in the sample heat-treated at 900 °C are 10-20 nm in size, distributed homogeneously among the amorphous silica matrix. The efficient upconversion emission for Er^3+.4F_9/2 → ⁴I_15/2 transition was recorded under 980 nm excitation, which could be ascribed to the incorporation of Er³⁺ ions into the CaF₂ nano-crystals with low phonon energy.     

A comparative study on the NO2 gas sensing properties of ZnO thin films, nanowires and nanorods

ZnO thin films were fabricated using the spin coating method, ZnO nanowires by cathodically induced sol-gel deposition by the means of an anodic aluminum oxide (AAO) template, and ZnO nanorods with the hydrothermal technique. For thin film preparation, a clear, homogeneous and stable ZnO solution was prepared by the sol-gel method using zinc acetate (ZnAc) precursor which was then coated on a glass substrate with a spin coater. Vertically aligned ZnO nanowires which were approximately 65 nm in diameter and 10 μm in length were grown in an AAO template by applying a cathodic voltage in aqueous zinc nitrate solution at room temperature. For fabrication of the ZnO nanorods, the sol-gel ZnO solution was coated on glass substrate by spin coating as a seed layer. Then ZnO nanorods were grown in zinc nitrate and hexamthylenetetramine aqueous solution. The ZnO nanorods are approximately 30 nm in diameter and 500 nm in length. The ZnO thin film, ZnO nanowires and nanorods were characterized by X-ray diffraction (XRD) analysis and scanning electron microscope (SEM). The NO₂ gas sensing properties of ZnO thin films, nanowires and nanorods were investigated in a dark chamber at 200 °C in the concentration range of 100 ppb-10 ppm. It was found that the response times of both ZnO thin films and ZnO nanorods were approximately 30 s, and the sensor response was depended on shape and size of ZnO nanostructures and electrode configurations.     

Influence of annealing treatments for ZnO-doped Zr0.8Sn0.2TiO4 thin films by RF magnetron sputtering

Zirconium tin titanium oxide doped 1 wt.% ZnO thin films on n-type Si substrate were deposited by rf magnetron sputtering at a fixed rf power of 300 W, a substrate temperature of 450 °C, a deposition pressure of 5 mTorr and an Ar/O₂ ratio of 100/0 with various annealing temperatures and annealing times. Electrical properties and microstructures of 1 wt.% ZnO-doped (Zr_0.8Sn_0.2)TiO₄ thin films prepared by rf magnetron sputtering on n-type Si(100) substrates at different annealing temperatures (500 °C-700 °C) and annealing times (2 h-6 h) have been investigated. The structural and morphological characteristics analyzed by X-ray diffraction (XRD) and atomic force microscope (AFM) were sensitive to the treatment conditions such as annealing temperature and annealing time. At an annealing temperature of 600 °C and an annealing time of 6 h, the ZnO-doped (Zr_0.8Sn_0.2)TiO₄ thin films possess a dielectric constant of 46 (at f = 10 MHz), a dissipation factor of 0.059 (at f = 10 MHz), and a low leakage current density of 3.8 × 10^− 9 A/cm² at an electrical field of 1 kV/cm.     

Effect of thickness of ZnO active layer on ZnO-TFT's characteristics

We have investigated the electrical characteristics of ZnO thin film transistors with respect to the thickness of ZnO active layers. The ZnO layers with the thickness of 30 nm to 150 nm were deposited on bottom gate patterned Si substrate by RF sputtering at room temperature. The low-temperature oxide served as gate dielectric. As ZnO channel layer got thicker, the leakage current at V_DS = 30 V and V_G = 0 V greatly increased from 10^− 10 A to 10^− 6 A, while the threshold voltage decreased from 15 V to 10 V. On the other hand, the field effect mobility got around 0.15 cm²/V s except for the 30-nm-thick channel. Overall, the 55-nm-thick ZnO channel layer showed the best performance.     

The effect of MgO and SiO2 codoping on the properties of Nd:YAG transparent ceramic

Nd:YAG transparent ceramics were fabricated by a reactive sintering method under vacuum using SiO₂, MgO and compound additives (SiO₂ and MgO) as sintering aids. The effects of SiO₂ and MgO on the microstructure and sintering process of Nd:YAG ceramics were studied. High quality Nd:YAG ceramics with compound sintering aids obtained by vacuum sintering at 1780 °C are composed of grains of the size ∼10 μm, and their transmittance is 82% at 400 nm. It was found the absorption coefficient of 1.0 mol% Nd:YAG ceramic was 8.6 cm⁻¹ at 808 nm and its absorption cross section was calculated to be 6.26 × 10⁻²⁰ cm².     

Large third-order optical nonlinearity of SrBi2Nb2O9 thin films fabricated by pulsed laser deposition

SrBi₂Nb₂O₉ (SBN) thin films with a single phase of layered perovskite structure have been fabricated on fused quartz substrates at room temperature by pulsed laser deposition. The XRD and AFM analysis indicated that the films had better crystallinity, less rough surface morphology, and larger grain size with increasing oxygen pressure. The nonlinear optical properties of the samples were determined using a single beam Z-scan technique at a laser wavelength of 532 nm with laser duration of 25 ps. The real and imaginary parts of the third-order nonlinear optical susceptibility χ⁽³⁾ of the films were measured to be 3.18 × 10^− 8 esu and 5.94 × 10^− 9 esu, respectively.     

Nanostructured solar cell by spray pyrolysis: Effect of titania barrier layer on the cell performance

ZnO nanostructured solar cells with CuInS₂ absorber layer were prepared by chemical spray method. In order to increase chemical stability of ZnO nanorods against dissolution in the next steps of the cell preparation, and reduce the electrical shorts between the front and back contacts, an amorphous TiO₂ layer was deposited on ZnO nanorods by ALD or sol-gel spray technique. The thicknesses of the layer (≤ 5 nm by spray and ≤ 1 nm by ALD), which did not impede the collection of carriers, were determined. TiO₂ thicknesses above these optimal values led to s-shaped I-V curves, causing the decrease in solar cell efficiency from 2.2 to 0.7% due to the formation of an additional junction blocking charge carrier transport in the device under forward bias. Nanostructured cells suffered from somewhat higher interface recombination but showed still two times higher current densities (~ 10 mA/cm²) than the planar devices did.     

Silicon metal-semiconductor-metal photodetector with zinc oxide transparent conducting electrodes

Transparent conducting oxides thin layers, due to their optical and electrical properties, can be used as transparent electrodes in various optoelectronic devices. We present a metal-semiconductor-metal photodiode (MSM-PD) on silicon as optically active layer with zinc oxide (ZnO) thin layer as interdigitated Schottky transparent electrodes. The advantage of using a ZnO thin layer as Schottky electrodes consists in the improvement of the photoresponse by eliminating the shadowing of the active area by opaque metallic electrodes. ZnO thin layers were deposited on 10 Ω cm resistivity silicon epitaxial wafers by the vacuum thermal evaporation method. High purity metallic powders were mixed with an (Al + Sn)/Zn ratio of 0.03. In order to obtain transparent layers the metallic depositions were thermally treated at 450 °C for 2 h. The Al, Sn co-doped ZnO layers of 0.5-0.8 μm were investigated regarding structural, optical and electrical properties and surface morphology. The obtained thin layers have a high transparency (T > 85%) over a large spectral range and the resistivity is quite low, ρ ~ 10^− 4 Ω cm. The interdigitated Schottky contacts of ZnO were configurated onto the optically active Si layer providing an MSM-PD structure of 0.143 mm² active area and finger spacing and finger width of 6 μm. The optoelectronic characteristics were measured and the Schottky barrier height of 0.62 eV was determined from the current-voltage characteristic. A responsivity of 0.2 A/W at 475 nm and a capacitance of 1.4 pF at 10 V bias were obtained for the MSM-PD structure with transparent conducting ZnO Schottky electrodes.     

Preparation and laser performance of Nd-doped yttrium lanthanum oxide transparent ceramic

1.5 at.% Nd:Y_1.8La_0.2O₃ transparent ceramic was fabricated by a solid-state reaction method and sintered at 1650-1700 °C for 40-50 h under H₂ atmosphere. The spectroscopic properties were investigated at room temperature. The transparent ceramic has excellent spectroscopic properties, with the absorption cross section of 1.50 × 10⁻²⁰ cm² and broad full width at half maximum (FWHM) of about 8 nm at LD wavelength 806 nm, the emission cross section of 2.03 × 10⁻²⁰ cm² at 1079 nm, and the decay lifetime of 200 μs. Laser performance was carried out using an uncoated Nd:Y_1.8La_0.2O₃ ceramic plate under laser diode end-pumping without any water cooling device. The room temperature thermal conductivity of this ceramic is 6.20 W/mK. For Nd:Y_1.8La_0.2O₃ ceramic laser, a maximum output power of 62 mW was obtained at 1079 nm under a 808 nm diode pump.     

Rapid synthesis of pure and narrowly distributed Eu doped ZnO nanoparticles by solution combustion method

Pure ZnO:Eu³⁺ nanoparticles (~ 50 nm) were prepared by a solution combustion method. ZnO and Eu₂O₃ were used as starting materials and dissolved in nitric acid. Citric acid was used as a fuel. The reaction mixture was heated at 350 °C resulting into a rapid exothermic reaction yielding pure nanopowders. The atomic weight concentration of Eu³⁺ doped in ZnO was 20%. Transmission electron microscopy (TEM) was used to study the particle size and morphology. The nanopowders were characterized for phase composition using X-ray diffractrometry (XRD). Particle size distribution (PSD) analysis of ZnO: Eu³⁺ showed particle sizes ranging from 30 to 80 nm.The photoluminescence emission spectra of ZnO:Eu³⁺ nanostructures showed a strong band emission around 618 nm when excited with 515 nm wavelength.     

Investigation of low resistance transparent MoO3/Ag/MoO3 multilayer and application as anode in organic solar cells

Depending on the resistivity and transmittance, transparent conductive oxides (TCO) are widely used in thin film optoelectronic devices. Thus doped In₂O₃ (ITO), ZnO, SnO₂ are commercially developed. However, the deposition process of these films need sputtering and/or heating cycle, which has negative effect on the performances of the organic devices due to the sputtering and heat damages. Therefore a thermally evaporable, low resistance, transparent electrode, deposited onto substrates room temperature, has to be developed to overcome these difficulties. For these reasons combination of dielectric materials and metal multilayer has been proposed to achieve high transparent conductive oxides. In this work the different structures probed were: MoO₃ (45 nm)/Ag (x nm)/MoO₃ (37.5 nm), with x = 5-15 nm. The measure of the electrical conductivity of the structures shows that there is a threshold value of the silver thickness: below 10 nm the films are semiconductor, from 10 nm and above the films are conductor. However, the transmittance of the structures decreases with the silver thickness, therefore the optimum Ag thickness is 10 nm. A structure MoO₃ (45 nm)/Ag (10 nm)/MoO₃ (37.5 nm) resulted with a resistivity of 8 × 10^− 5 Ω cm and a transmittance, at around 600 nm, of 80%. Such multilayer structure can be used as anode in organic solar cells according to the device anode/CuPc/C₆₀/Alq₃/Al. We have already shown that when the anode of the cells is an ITO film the introduction of a thin (3 nm) MoO₃ layer at the interface anode (ITO)/organic electron donor (CuPc) allows reducing the energy barrier due to the difference between the work function of ITO and the highest occupied molecular orbital of CuPc [1]. This property has been used in the present work to achieve a high hole transfer efficiency between the CuPc and the anode. For comparison MoO₃/Ag/MoO₃/CuPc/C₆₀/Alq₃/Al and ITO/MoO₃/CuPc/C₆₀/Alq3/Al solar cells have been deposited in the same run. These devices exhibit efficiency of the same order of magnitude.     

Characteristics of transparent ZnO based thin film transistors with amorphous HfO2 gate insulators and Ga doped ZnO electrodes

Coplanar type transparent thin film transistors (TFTs) have been fabricated on the glass substrates. The devices consist of intrinsic ZnO, Ga doped ZnO (GZO), and amorphous HfO₂ for the semiconductor active channel layer, electrode, and gate insulator, respectively. GZO and HfO₂ layers were prepared by using a pulsed laser deposition (PLD) and intrinsic ZnO layers were fabricated by using an rf-magnetron sputtering. The transparent TFT exhibits n-channel, enhancement mode behavior. The field effect mobility, threshold voltage, and a drain current on-to-off ratio were measured to be 14.7 cm²/Vs, 2 V, and 10⁵, respectively. High optical transmittance (> 85%) in visible region makes ZnO TFTs attractive for transparent electronics.     

Synthesis of MoO3 nanobelts by medium energy nitrogen ion implantation

Ion implantation has been revealed as a potential technique to modify the surface of materials. In this work, MoO₃ nanobelts were synthesized on MoO₃ whisker surfaces by means of ion implantation with 60 keV nitrogen ions at a dose of 1 × 10¹⁶ atom/cm² and characterized by scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy. The result showed that the nanostructures of MoO₃ occurred over the whisker surfaces and had belt-like shapes. The size of the synthesized MoO₃ nanobelts mostly ranged from 20 to 60 nm in width and 300 to 800 nm in length. The nanobelts were found to have an orthorhombic crystal structure with growth preferential in the [001] direction. The growth process of the nanobelts based on the common vapor-solid mechanism is discussed.